In copper extraction, where the employed extraction solution is for instance hydroxyoxime dissolved into kerosene, it has been proved that the linear flow speed of the organic solution must not surpass the speed of 45-60 mm/s. In other cases the volume of entrainment, i.e. the amount of small water drops left in the extraction solution, begins to grow to a disturbing extent, even to the order of 1000 ppm, when a conventional separation method in a simple settler is applied. The linear speed can be decreased only to a limited extent by increasing the layer volume of the organic solution, because when the layer volume is increased, there is respectively needed more of the expensive organic reagent. This problem is generally solved so that there is used a 250-350 mm thick layer of organic solution, and the linear speed of the extraction solution is reduced by adding the width of the flow field of the separable solutions. Nowadays the width of the employed settler may be as large as its length, even larger. When the total volume of the solution flow is about 2,000 m.sup.3 /h, the settler width is of the order 25 m and its area 600-800 m.sup.2.
The Finnish patent application 93 5393 introduces a method and apparatus whereby the pumping and mixing units in extraction plants can be improved, so that the previous maximum flow can be increased to be even 2.5-fold. In addition to pumping and mixing, said method and apparatus also relate to conducting the dispersion flow from the mixer into the settler, to the shape of the adjusting gate and picket fence arranged at the front end of the settler, as well as to the separation of organic solution at the final end of the settler.
According to a prevailing notion, the linear speed difference between the extraction solution and the aqueous solution must be small in order to avoid a remixing of the solutions owing to boundary surface turbulence and a resulting increase in the entrainment levels. As a solution for this problem, there are used low separation tanks, where the aqueous solution layer is not essentially thicker than the extraction solution layer. For instance in traditional copper extraction, the employed thickness of the aqueous solution is 400-450 mm, and the thickness of the corresponding extraction solution layer is 250-350 mm, so that the ratio of the thicknesses of the extraction and aqueous solution layers, particularly at the discharge end of the settler, is 1:1.5, at highest 1:1.7. When copper extraction is run so that in the mixer the ratio of the volume flows of the solution feed is 1:1, this results in that the linear speed of the extraction solution sets on a level which is 40-80%, generally 50-70% higher than the linear speed of the solution.